Alternate triggering in digital oscilloscopes

ABSTRACT

A digital storage oscilloscope includes a source of two or more input signals and a display device. An alternate triggering mechanism includes a selector for selecting one of the input signals. A trigger circuit is responsive to the selected input signal, and generates a trigger signal when a trigger condition is satisfied. A raster signal generator includes a plurality of waveform memories each associated with a respective input signal and responsive to the trigger signal for storing data representing the selected input signal in the waveform memory associated with the selected input signal. The raster signal generator generates a raster image signal representing the selected input signal.

FIELD OF THE INVENTION

The present invention relates to triggering mechanisms and associateddisplay modes in a digital storage oscilloscope

BACKGROUND OF THE INVENTION

Prior analog oscilloscopes included a triggering mode known as alternatetriggering. In this mode of operation, traces of respective inputsignals were alternately displayed on the face of a display device, eachtriggered when it is displayed. The persistence of the phosphors on thescreen of the display tube retained the image of each signal long enoughfor the signals to appear to be displayed stably and simultaneously.

FIG. 1 is a block diagram of a portion of a prior art analogoscilloscope having this triggering mode. In FIG. 1, input terminals areillustrated for two input signals, also called channel signals, CH1 andCH2. The input terminals CH1 and CH2 may, for example, be oscilloscopeprobes, which are connected to circuit nodes producing signals which theuser wishes to observe. The channel 1 input terminal CH1 is coupled toan input terminal of a first input amplifier 12 and the channel 2 inputterminal CH2 is coupled to an input terminal of a second input amplifier14. An output terminal of the first input amplifier 12 is coupled torespective first data input terminals of a first analog multiplexer 20and a second analog multiplexer 30. An output terminal of the secondinput amplifier 14 is coupled to respective second data input terminalsof the first analog multiplexer 20 and the second analog multiplexer 30.An output terminal of the first analog multiplexer 20 is coupled to aninput terminal of a vertical drive amplifier 40. Differential outputterminals of the vertical drive amplifier 40 are coupled to respectivevertical deflection plates 52 of a display tube 50.

An output terminal of the second analog multiplexer 30 is coupled to afirst input terminal of a trigger circuit 60. In FIG. 1 the triggercircuit 60 is illustrated as a comparator, however one skilled in theart will understand that other triggering circuits were available inanalog oscilloscopes as illustrated in FIG. 1. A source of a thresholdsignal, Th, is coupled to a second input terminal of the trigger circuit60. An output terminal of the trigger circuit 60 is coupled to an inputterminal of a sweep signal generator 70. Differential output terminalsof the sweep signal generator are coupled to respective horizontaldeflection plates 54 of the display tube 50. A status output terminal ofthe sweep signal generator 70 is coupled to an input terminal of aswitch control circuit 80. Respective control output terminals of theswitch control circuit 80 are coupled to control input terminals of thefirst analog multiplexer 20 and the second analog multiplexer 30

In operation, the first and second analog multiplexers 20 and 30 aresimultaneously conditioned to couple to their output terminals aselected channel input signal, that is, either the channel 1 inputsignal CH1 or the channel 2 input signal CH2, under the control of theswitch control circuit 80. For the following description, it will beassumed that the input signal initially selected is the channel 1 inputsignal CH1. Thus, the first analog multiplexer 20 couples the channel 1input signal CH1 to the vertical drive amplifier 40 and the secondanalog multiplexer 30 couples the channel 1 input signal CH1 to thetrigger circuit 60.

The trigger circuit 60 compares the channel 1 input signal, CH1, to thethreshold amplitude Th. When the amplitude of the channel 1 input signalpasses through the threshold amplitude Th, the trigger circuit 60generates a signal at its output terminal which conditions the sweepsignal generator 70 to produce a ramp signal. This ramp signal isapplied to the horizontal deflection plates 54. The ramp signal at thehorizontal deflection plates 54 produces a single sweep of the electronbeam from the left edge to the right edge of the display screen, takinga user controlled period of time.

Simultaneously, the channel 1 input signal CH1 is amplified by thevertical drive amplifier 40 and applied to the vertical deflectionplates 52. The signal at the vertical deflection plates 52 causes thevertical position of the electron beam sweep to vary as a function ofthe channel 1 signal amplitude. The combination of the verticaldeflection plates 52 and the horizontal deflection plates 54 produce onevisible trace on the face of the display tube 50 of the waveformrepresenting the amplitude of the channel signal 1 over the time periodspecified by the sweep signal generator 70 starting at the time thetrigger circuit detected the trigger condition.

When the horizontal sweep is completed, the control signal from thesweep signal generator 70 signals the switch control circuit 80 tocondition the first and second analog multiplexers 20 and 30 to selectthe other channel input signal. That is, the first and second analogmultiplexers 20 and 30 now both couple the channel 2 input signal, CH2,to their output terminals. The trigger circuit 60 triggers on theamplitude of the channel 2 input signal CH2 initiating a sweep via thesweep signal generator 70, and the vertical deflection plates 52 changethe vertical position of the sweep according to the amplitude of thischannel signal. The display device 50, thus, displays one trace of thechannel 2 input signal CH2. When the sweep is complete, the switchcontrol circuit 80 conditions the first and second analog multiplexers20 and 30 to couple the channel 1 input signal CH1 to the vertical driveamplifier 40 and trigger circuit 60 once more, and the process beginsagain. Traces of the channel 1 signal and the channel 2 signal, eachtriggered by its own amplitude, are alternated on the face of thedisplay tube 50 in this mode of operation, appearing stably andsimultaneously due to phosphor persistence.

Digital oscilloscopes operate very differently from analogoscilloscopes. FIG. 2 is a block diagram of a known digital storageoscilloscope. In FIG. 2, elements which are the same as thoseillustrated in FIG. 1 are designated by the same reference numbers andare not described in detail below.

In FIG. 2, the output terminal of the channel 1 input amplifier 12 iscoupled to an input terminal of a first A/D converter 122 and to a firstinput terminal of a trigger circuit 180, and more specifically to afirst input terminal of a signal selector 182 in the trigger circuit180. The output terminal of the channel 2 input amplifier 14 is coupledto an input terminal of a second A/D converter 124 and a second inputterminal of the trigger circuit 180, and more specifically to a secondinput terminal of the signal selector 182. An output terminal of thefirst A/D converter 122 is coupled to an input terminal of a firstacquisition memory 132 and an output terminal of the second A/Dconverter 124 is coupled to an input terminal of a second acquisitionmemory 134. An output terminal of the first acquisition memory 132 iscoupled to an input terminal of a first unwrap circuit 142, and anoutput terminal of the second acquisition memory 134 is coupled to aninput terminal of a second unwrap circuit 144. An output terminal of thefirst unwrap circuit 142 is coupled to an input terminal of a firstwaveform memory 152 and an output terminal of the second unwrap circuit144 is coupled to an input terminal of a second waveform memory 154. Anoutput terminal of the first waveform memory 152 is coupled to a firstinput terminal of a rasterizer 160 and an output terminal of the secondwaveform memory 154 is coupled to a second input terminal of therasterizer 160. An output terminal of the rasterizer 160 is coupled toan input terminal of a display device 170.

An output terminal of the trigger circuit 180 is coupled to a controlsequencer 190. Respective control signal output terminals of the controlsequencer 190 are coupled to corresponding input terminals of the firstand second A/D converters, 122 and 124, the first and second unwrapcircuits, 142 and 144, and the rasterizer 160. A status output terminalof the control sequencer 190 is coupled to a corresponding inputterminal of a central processing unit (CPU) 200. Respective controloutput terminals of the CPU 200 are coupled to corresponding inputterminals of the control sequencer 190 and the trigger circuit 180.

The digital oscilloscope in FIG. 2 is illustrated as including only twoinput signal channels. One skilled in the art, however, will understandthat more than two channels may be simultaneously processed in the samemanner. More specifically, digital storage oscilloscopes including fourinput signal channels are widely available. In addition, in FIG. 2, theacquisition memories 132 and 134 and the waveform memories 152 and 154are illustrated as separate memories. However, one skilled in the artwill understand that these memories may be implemented by allocatingseparate portions of a single larger memory to these differentfunctions.

In operation, the first and second A/D converters 122 and 124 generaterespective digital sample sequences representing the channel 1 andchannel 2 input signals CH1 and CH2, under the control of the controlsequencer 190. These sample sequences are stored in their respectiveacquisition memories 132 and 134. The trigger circuit 180 generates atrigger signal when an input signal or a combination of input signals atits input terminals satisfies a user specified trigger condition. Thetrigger circuit 180 sends the trigger signal to the control sequencer190, which determines which of the samples stored in the respectiveacquisition memories corresponds to the left edge of the desiredwaveform traces depending on the timing of the trigger signal. Thecontrol sequencer 190 controls the unwrap circuits 142 and 144 toextract the samples from the respective acquisition memories 132 and 134representing one trace of all of the signals, starting with therespective samples representing the left edges of the traces, and storesthose samples in the respective waveform memories 152 and 154. Therasterizer 160 retrieves the samples from the waveform memories 152 and154 and generates a bit mapped raster image of waveforms representingall the channel input signals (i.e. the channel 1 and 2 input signalsCH1 and CH2). This bit mapped raster image is then displayed on thedisplay device 170, which may, for example, be a liquid crystal display(LCD). It will be understood that every raster signal generated by therasterizer 160, and displayed on the display device 170, includes tracesfor every input channel signal, derived from the sample sequencescorresponding to those signals generated by the A/D converters 122 and124.

Referring to the trigger circuit 180, it can be seen that all of thetrigger signals, i.e. the channel input signals CH1 and CH2 and allother channel input signals (not shown), are coupled to the triggercircuit 180 through the signal selector 182. It is also possible for anexternal signal (also not shown) to be coupled to an input terminal ofthe trigger circuit 180 through the signal selector 182. The signalselector 182 selects from among all the input signals those signalsanalyzed by the trigger circuit 180 to generate the trigger signal. Thetrigger circuit 180 monitors the selected signals from the signalselector 182 and when they meet a user supplied trigger condition, assupplied by the CPU 200 in response to user signals (also not shown),supplies the trigger signal to the control sequencer 190. It will beunderstood from FIG. 2 that the trigger condition, once set by the user,remains unchanged, and all traces produced by the rasterizer 160 areproduced in response to this trigger condition being met by the inputsignals.

In a digital oscilloscope, the control sequencer 190 provides thelocation of the sample forming the left side (beginning) of the trace tothe unwrap circuits 142 and 144 when the trigger criteria have been met,instead of starting a trace at that time, as in analog oscilloscopes. Adigital oscilloscope, consequently, can provide much more sophisticatedtriggering and display modes than an analog oscilloscope can. Forexample, pre and post triggering and delayed triggering may all beprovided by a digital oscilloscope.

However, it is sometimes desirable to use the advanced triggering anddisplay capabilities of a digital oscilloscope to alternately display afirst waveform using a first trigger condition and a second waveformusing a second trigger condition. For example, some forms ofsynchronized stereophonic sound signals on movie film consists of twosound tracks recorded on opposite sides of the film. Under idealcircumstances, the two sound signals extracted from these sound tracksshould remain in synchronism. However, due to the mechanical nature ofthe playback projector, these sounds tracks often lose synchronism.Regardless, it is desirable to observe these two recovered sound signalsboth as a stable display, i.e. each triggered by its own amplitude.

BRIEF SUMMARY OF THE INVENTION

In accordance with principles of the present invention a digital storageoscilloscope includes a source of two or more input signals and adisplay device. An alternate triggering mechanism includes a selectorfor selecting one of the input signals. A trigger circuit is responsiveto the selected input signal, and generates a trigger signal when atrigger condition is satisfied. A raster signal generator includes aplurality of waveform memories each associated with a respective inputsignal and responsive to the trigger signal for storing datarepresenting the selected input signal in the waveform memory associatedwith the selected input signal. The raster signal generator generates araster image signal representing the selected input signal.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a block diagram of a prior art analog oscilloscope;

FIG. 2 is a block diagram of a prior art digital oscilloscope; and

FIG. 3 is a block diagram of a digital storage oscillator according toprinciples of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 is a block diagram of a digital storage oscilloscope according toprinciples of the present invention. In FIG. 3, those elements which arethe same as those illustrated in FIG. 2 are designated by the samereference number and are not described in detail below.

In FIG. 3, the output terminal from the first input amplifier 12 iscoupled to a first input terminal of an analog multiplexer 110 and theoutput terminal from the second input amplifier 14 is coupled to asecond input terminal of the analog multiplexer 110. In the illustratedembodiment, the analog multiplexer 110 is implemented as a part of thesignal selector 182 within the trigger circuit 180, and is illustratedin phantom. An output terminal of the analog multiplexer 110 is coupledto the remainder of the circuitry in the trigger circuit 180. Thetrigger circuit 180 may be any of a number of known designs. A controloutput terminal of the CPU 200 is coupled to the control input terminalof the analog multiplexer 110.

In the illustrated embodiment, the signal selector 182 is controlled bythe CPU in such a manner as to operate in the same way as an analogmultiplexer 110 would operate. Thus, in operation, the CPU 200 controlsthe analog multiplexer 110 to couple one of the input channel signals,CH1 or CH2, to the trigger circuit 180. For example, assume that the CPU200 initially conditions the analog multiplexer 110 to couple thechannel 1 input signal CH1 to the trigger circuit 180. In this case, thetrigger signal from the trigger circuit 180 is generated based on thechannel 1 input signal CH1. However, all the known sophisticatedtriggering and display capabilities of the digital oscilloscope may beused to generate the trigger signal, e.g. pre and post trigger and/ordelayed triggering.

When a trigger signal is generated by the trigger circuit 180, thecontrol sequencer 190 conditions the first unwrap circuit 142 toretrieve the desired samples representing the channel 1 input signal CH1from the first acquisition memory 132 and store them in the firstwaveform memory 152. At the same time, the second unwrap circuit 144 isconditioned to not retrieve samples representing the second channelinput signal CH2 from the second acquisition memory 134. Thus, thesecond waveform memory 154 is not updated with new samples representingthe channel 2 input signal CH2. Instead the previous samples remain inthe second waveform memory 154.

The rasterizer 160 retrieves the waveform representative samples fromthe waveform memories 152 and 154 and generates a raster image signalrepresenting the waveforms corresponding to those samples. The displaydevice 170 then displays the raster image represented by that signal.Consequently, the waveform image displayed by the display device 170 isbased on the newly received samples representing the channel 1 inputsignal CH1 and the previously received samples representing the channel2 input signal CH2.

When the rasterizer 160 has generated the raster image signalrepresenting the waveforms, as described above, the CPU 200 conditionsthe analog multiplexer 110 to couple the channel 2 input signal CH2 tothe trigger circuit 180. In this case, the trigger signal from thetrigger circuit 180 is generated based on the channel 2 input signalCH2. When a trigger signal is generated by the trigger circuit 180, thecontrol sequencer 190 conditions the second unwrap circuit 144 toretrieve the desired samples representing the channel 2 input signal CH2from the second acquisition memory 134 and store them in the secondwaveform memory 154. At the same time, the first unwrap circuit 142 isconditioned to not retrieve samples representing the channel 1 inputsignal CH1 from the first acquisition memory 132. Thus, the firstwaveform memory 152 is not updated with new samples representing thechannel 1 input signal CH1. The rasterizer 160 retrieves the waveformrepresentative samples from the waveform memories 152 and 154 andgenerates a raster image signal representing the waveforms correspondingto those samples. The display device 170 then displays the raster imagerepresented by that signal. Consequently, the waveform image generatedby the rasterizer 160 is based on the previously received samplesrepresenting the channel 1 input signal CH1 and the newly receivedsamples representing the channel 2 input signal CH2.

When the rasterizer 160 has generated the raster representing thewaveforms as described above, the CPU 200 conditions the analogmultiplexer 110 to couple the channel 1 input signal CH1 to the triggercircuit 180 again, and the process repeats. In this manner, a waveformdisplay which operates similarly to the alternate triggering mode inanalog oscilloscopes is generated. This alternate triggering mode may beused to simultaneously display two signals, each triggered by its ownamplitude, so that the waveforms appear stable on the display device,even though the signals may not be in synchronism.

In one embodiment, both the first and second A/D converters 122 and 124operate to digitize the signals at their input terminals to generatesamples representing the channel 1 input signal CH1 and channel 2 inputsignal CH2 respectively and store them in the corresponding acquisitionmemories 132 and 134. In another embodiment, only the A/D converterwhose channel signal is supplied to the trigger circuit 180 through theanalog multiplexer 110 operates to generate samples and store them inthe corresponding acquisition memory.

While the preferred embodiment has been described for a digitaloscilloscope including two channel input signals, one skilled in the artwill understand that any number of input signals may be included. Allthe input signals are coupled to corresponding input terminals of theanalog multiplexer 110, and when the alternate triggering mode describedabove is enabled, all the input signals are sequentially coupled to thetrigger circuit 180 under control of the CPU 200.

What is claimed is:
 1. In a digital storage oscilloscope including asource of two or more input signals and a display device, an alternatetriggering mechanism, comprising: a selector, coupled to the inputsignal source, for sequentially selecting respective ones of the inputsignals; a trigger circuit, responsive to the selected input signal, forgenerating a trigger signal when a trigger condition is satisfied; and araster signal generator, coupled between the input signal source and thedisplay device, and including a plurality of waveform memories eachassociated with a respective input signal and responsive to the triggersignal for storing data representing the selected input signal in thewaveform memory associated with the selected input signal, the rastersignal generator generating an image signal representing the waveform ofthe signals represented by the data in the plurality of waveformmemories.
 2. The mechanism of claim 1 wherein the selector comprises ananalog multiplexer.
 3. The mechanism of claim 1 wherein the rastersignal generator comprises: a plurality of digitizers, each responsiveto a respective input signal for generating a sequence of samplesrepresenting the respective input signal; a plurality of waveform sampleselectors, coupled to respective digitizers and responsive to thetrigger signal, for selecting samples representing a waveform of theselected input signal and storing the selected samples in the waveformmemory associated with the selected input signal; and a rasterizer,coupled to the plurality of waveform sample selectors for generating theraster image signal representing at least the waveform corresponding tothe selected input samples for the selected input signal.
 4. Themechanism of claim 3 wherein each of the plurality of digitizerscomprises: an analog to digital converter, responsive to the respectiveinput signal, for generating the sample sequence; and an acquisitionmemory, coupled to the analog to digital converter, for storing thesample sequence.
 5. The mechanism of claim 4 wherein each of thewaveform sample selectors comprises: a waveform circuit, coupled to theacquisition memory and responsive to the trigger circuit, for retrievingsamples representing the waveform of the respective input signal if therespective input signal is the selected input signal and storing theselected samples in the waveform memory associated with the selectedinput signal.
 6. The mechanism of claim 5 wherein the rasterizer iscoupled to the plurality of waveform memories and generates the rasterimage signal representing the waveforms corresponding to the storedinput samples for all of the input signals.
 7. The mechanism of claim 3wherein each of the plurality of waveform sample selectors comprises: awaveform circuit, coupled to the respective digitizer and responsive tothe trigger signal, for selecting samples representing the waveform ofthe respective input signal if the respective input signal is theselected input signal and storing the selected samples in the waveformmemory associated with the selected input signal.
 8. The mechanism ofclaim 7 wherein the rasterizer is coupled to the plurality of waveformmemories and generates the raster image signal representing thewaveforms corresponding to the stored selected input samples for all ofthe input signals.
 9. The mechanism of claim 1 further comprising acentral processing unit (CPU), responsive to user inputs, and coupled tothe selector and raster image signal generator, for controlling theoperation of the alternate triggering mechanism.
 10. In a digitalstorage oscilloscope including a source of at least two input signals,and a display device, a method for providing alternate triggering,comprising the steps of: sequentially selecting respective ones of theinput signals; generating a trigger signal, in response to the selectedinput signal, when a trigger condition is satisfied; storing datarepresenting the selected signal in response to the trigger signal;generating a raster image signal from newly stored data representing theselected input signal; and displaying an image represented by the imagesignal.
 11. The method of claim 10 wherein the raster image signalgenerating step further comprises the step of generating the rasterimage signal from previously stored data represented non-selected inputsignals.